Cork stoppers have been used as a stopper for sealing a beverage container such as a wine bottle through the ages because they exhibit good seal performance and have unique texture.
Compressed cork stoppers produced by compression molding cork grains and an adhesive have been extensively used since they are advantageous in cost or the like and advantageous in the environment because cork grains obtained by pulverizing scraps generated during producing a natural-cork stopper can be used as a starting material. However, there have been the problems that a liquid as a content may permeate a compressed cork stopper and gas barrier performance may be insufficient, and thus improvement of sealability has been needed.
Various techniques have been developed for solving such problems. Patent Reference No. 1 has described a stopper produced by placing cork powder, thermally expandable microspheres containing an expansion agent consisting of hydrocarbon such as isobutane in an outer shell made of a copolymer of methyl methacrylate and acrylonitrile, a polyurethane adhesive and so on, in a mold, and then heating the mixture. However, a cork stopper obtained may be insufficient in sealability. Furthermore, a content may be contaminated with acrylonitrile contained in the microspheres, which is also problematic in safety. Patent Reference No. 2 has described a method for producing a cork stopper by compressing a mixture of cork grains and a binder resin along a direction perpendicular to a longitudinal direction of the cork stopper when the mixture is compression molded to provide a columnar cork stopper. However, a cork stopper obtained may be insufficient in sealability.